1. Field of the Invention
This invention provides a process for converting biomass to useful chemicals or fuels, by anaerobic fermentation of biomass and recovery of useful products from the fermentation medium. By alternative arrangement of the process steps of this invention, a variety of products (i.e. organic acids, ketones, aldehydes, and alcohols) may be produced from biomass. These products are made from salts of the organic acids (e.g., acetate, propionate, butyrate, lactate) that are the primary fermentation products of the fermentation.
2. Review of Related Art
Organic acids are important chemicals of commerce. Historically, organic acids were produced from animal fat or vegetable oil sources or from petroleum sources in substantially nonaqueous systems. More recently, organic acids have been identified as among the most attractive products for manufacture from biomass by fermentation. Biomass can be defined as any animal- or plant-based material of carbohydrate, protein or fat composition. Among the readily available sources of biomass are municipal solid waste (MSW) and sewage sludge (SS). At present, great expenditures of public funds are used to dispose of such wastes, including costs involved in treatment, transport, incineration, or dumping in landfills or oceans. The recovery of valuable products from biomass such as MSW and SS could recover the costs of disposal as well as reduce reliance on nonrenewable fossil fuel resources which serve as feedstock for most industrial organic acid production. Fermentation, therefore, can convert renewable organic materials, now considered a costly waste, into valuable chemical commodities.
However, the acids are produced by the fermentation in dilute aqueous solutions, and recovery of the acids in pure form involves separation from a large quantity of water. This recovery introduces significant operating expense into the process, while the physical plant required to handle the large volumes of solution introduces significant capital expense. The combination of capital and operating expense has, until now, made production of organic acids from biomass uneconomical. Thus, there remains a need for a process that combines unit operations for fermentation, concentration and recovery of organic acids to take advantage of potential synergies obtainable from integrating these processes, thereby generating an economical process for conversion of biomass to useful products.
Ketones, aldehydes, and alcohols predominately are produced from petroleum and natural gas. Because fossil fuels are a finite resource, it is desirable to identify processes that use renewable resources, such as biomass. Biomass-based alcohol production is currently practiced using corn as feedstock; however, because corn has alternative use as food, the feedstock is necessarily costly making the ethanol product expensive. Experimental technologies are being developed in which extracellular enzymes, such as cellulase and hemicellulase, are added to lignocellulosic biomass to produce sugars that are subsequently fermented to ethanol. The primary challenges of this technology are to develop inexpensive sources of enzyme and to develop organisms that can ferment the variety of sugars to ethanol with high yields.
The technology described herein overcomes problems associated with the competing biomass-based technologies by employing mixed cultures of microorganisms that convert the many components of biomass (e.g., cellulose, hemicellulose, pectin, sugar, protein, fats) to organic acids that are subsequently converted to ketones, aldehydes, and alcohols using a variety of chemical steps. Further, the microorganisms produce their own enzymes, thus avoiding the need to add costly extracellular enzymes.
It is an object of this invention to provide a process for converting salts of volatile fatty acids, produced by anaerobic fermentation of biomass, into liquid fuels.
It is an object of this invention to provide an improved method for converting biomass to organic acids, ketones, aldehydes, and alcohols.
These and other objects are accomplished by one or more of the following embodiments of this invention.
In one embodiment, this invention provides a method for thermally converting volatile fatty acid (VFA) salts to ketones which comprises the steps of precipitating metal salts of volatile fatty acids (VFAs) from the fermentation liquor of an anaerobic fermentation, then recovering and drying precipitated metal salts of VFAs, mixing dry metal salts of VFAs with a heat transfer agent, preferably steel balls, glass balls or ceramic balls, more preferably hollow balls that are filled with a substance that melts at the temperature of thermal decomposition of VFAs, in an evacuated container, the hot heat transfer agent being sufficient to raise the temperature of the metal salts of VFAs to cause thermal decomposition, with the resulting formation of ketone-containing vapor and metal salt of carbonate; and then separating the ketone-containing vapor from the metal carbonate salt and heat transfer agent, and recovering liquid ketones by condensing the ketone-containing vapor. Preferably, the metal salts of VFAs are alkali metal or alkaline earth salts, more preferably, calcium salts.
In another embodiment, this invention provides a method for recovering low-molecular-weight aldehydes and ketones from fermentation liquor produced by anaerobic fermentation of biomass, which method comprises the steps of (a) concentrating salts of volatile fatty acids (VFAs) from fermentation liquor produced by anaerobic fermentation of biomass; (b) precipitating and drying the calcium salts of VFAs; (c) adding salts of formic acid; (d) mixing the dry calcium salts of VFAs and formic acid with a heat transfer agent, thereby causing thermal decomposition of the calcium salts of VFAs to form ketone-containing and aldehyde-containing vapor and calcium carbonate; (e) maintaining a vacuum in the container by condensing ketones and aldehydes from the ketone-containing and aldehyde-containing vapor and removing non-condensable vapor from the container; (f) removing a mixture of calcium carbonate and heat transfer agent from the container; and (g) separating the heat transfer agent from the calcium carbonate, reheating and recycling the heat transfer agent, and calcining the calcium carbonate in a lime kiln. Optionally, the separated calcium carbonate could be recycled directly to the fermentor without further processing.
In yet another embodiment, this invention provides a mixture of secondary alcohols produced from ketones obtained from precipitated calcium salts of volatile fatty acids (VFAs) produced by anaerobic fermentation of biomass; drying the precipitated calcium salts of VFAs; mixing dry calcium salts of VFAs with a hot heat transfer agent in an evacuated container, thereby causing thermal decomposition of the calcium salts of VFAs to form ketone-containing vapor and calcium carbonate; separating the ketone-containing vapor from the calcium carbonate and heat transfer agent by condensing a mixture of ketones from the ketone-containing vapor; and finally hydrogenating the mixture of ketones recovered from the fermentation liquor. Alternatively, a mixture of primary and secondary alcohols can be produced can be produced by adding a calcium salt of formic acid to the evacuated container.
In an alternative embodiment, volative fatty acids (VFAs) can be produced by (a) anaerobically digesting biomass to produce a dilute solution of salts of VFAs; (b) concentrating the VFA salts; (c) adding a low-molecular-weight tertiary amine and carbon dioxide causing calcium carbonate to precipitate; (d) adding a high-molecular-weight tertiary amine to the solution from step c; (e) distilling off the low-molecular-weight tertiary amine; and (f) thermally converting the high-molecular-weight tertiary amine/VFA complex to high-molecular-weight amine and VFA. Alternatively, lactic acid can be produced by thermally converting a low-molecular-weight amine/lactic acid complex to low-molecular-weight amine and lactic acid.